Pressure fitting, in particular a pressure relief and safety valve

ABSTRACT

The invention relates to a pressure fitting (1) having a flow channel (42), a valve cone (5), a sliding element (43), and a locking device (12). The sliding element (43) is disposed displaceably along a main axis (41) of the pressure fitting (1). The valve cone is disposed on one end (44) of the sliding element (43). The flow channel (42) is closable by the valve cone (5) and the locking device (12) can be moved into a locking position, so that in a position of the valve cone (5) that opens the flow channel (42) the locking device (12) engages the valve cone (5) and/or the sliding element (42) and keeps the valve cone (5) in the position that opens the flow channel (42). Thereby a pressure relief in a pressurized system, in particular in a pressurizer (47) of a pressurized water reactor, can be assured even to a pressureless state. The pressure fitting is particular useful as a pressure relief valve or as a combined pressure relief and safety valve.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to a pressure fitting with a flow channel, a valvecone, a sliding element, and a locking device, wherein

a) the sliding element is disposed displaceably along a main axis of thepressure fitting;

b) the valve cone is disposed on one end of the sliding element;

c) the flow channel is closable by the valve cone;

d) the locking element can be moved into a locking position, so that ina position of the valve cone that opens the flow channel the lockingdevice engages the valve cone and/or the sliding element and keeps thevalve cone in the position that opens the flow channel.

In all fields in technology in which containers or pipes are underpressure, pressure fittings are used, especially for pressure relief andpressure control. Pressure relief or pressure control can beaccomplished both during normal operation and for safety of a systemwith components that are under pressure. Pressure vessels especially,under existing technical rules, must frequently be secured againstoverpressure. Moreover, in a pressure vessel, especially a steam boiler,the possibility of a pressure relief, even one to be initiated by hand,is demanded. By means of timely and sufficient pressure relief, theperformance of flexible safety provisions can be improved. This isespecially advantageous in a nuclear power plant with a primary systemthat is under pressure and that has components to be protected, such asthe reactor core. A pressure relief of a pressure vessel which is underhigh internal pressure, an example being a nuclear reactor pressurevessel with an internal pressure of about 150 bar, to a pressure on theorder of magnitude of an ambient pressure, of 2 to 4 bar, for instance,or to a pressureless state must be provided for, depending on the designspecifications.

A safety valve for sealing off a hydraulic system that is under elevatedpressure is described in British Patents GB 199 541 and GB 808 577. Thesafety valve is designed such that if a critical pressure is exceeded, apressure reduction can be carried out. A common feature of both patentsis that a structural embodiment of the safety valve is described inwhich by displacement of a locking element along an axis at right anglesto the direction of the stroke of the valve piston of the safety valve,a locking of the valve cone in the position that opens the safety valveis attained. After an opening of the safety valve in accordance withspecifications, or in other words after a critical pressure inside thehydraulic system is exceeded, the safety valve can be kept open down toa lower system pressure.

The locking element described in GB 808 577 is a ball, which is pressedinto a groove of the valve piston via a piston that is prestressed by aspring. The piston that displaces the ball may be returned to its outsetposition either by a manually operated device or optionally by anelectromagnetic device. In all cases, the locking element is embodiedsuch that every time the safety valve opens, this safety valve isautomatically kept open. Remote control of the locking arrangement islimited merely to returning the locking element or in other wordsre-stressing the spring.

The locking element described in GB 199 541 has two pins, disposed onopposite sides of a valve piston, which each engage a groove extendingin the valve piston. The pins are pressed against the valve piston via aprestressed spring, so that force-locking engagement with the valvepiston exists, preventing the piston from moving in its lift direction.If the pressure in the hydraulic system is so that both the frictionalforce of the pins and the force of the closing spring are overcome, thenthe piston is moved in the reciprocating direction. As soon as therespective grooves in the valve piston have reached the level of thepins, the pins are automatically pressed into the grooves in a mannerthat is not controllable from outside. Locking of the valve piston in aposition that opens the valve is thereby attained. The prestressing ofthe springs of the pins can be regulated in such a way that at varioussystem pressures, re-closing of the valve as a consequence of theclosing force of the valve spring is attained.

Both GB 808 577 and GB 199 541 are limited solely to keeping a safetyvalve open with locking elements, which are moved along an axis at rightangles to the reciprocating direction of the valve piston and which areput by a prestressed spring, without the possibility of influence(control) from outside, each time the valve opens into a position suchthat the valve piston remains in a position that opens the valve.

GB 808 577 discloses a fitting that includes a locking device. Thisdevice can be undone again only manually. German Patent Disclosure DE597 161 discloses a fitting that can be locked with the aid of a pawl.The pawl is pressed into the closing position by a spring.

In those prior art fittings, the moving parts are powerfully acceleratedand braked. This can be ascribed to the fact that they are not moveduntil there is a relatively high fluid pressure and are then lockedabruptly.

SUMMARY OF THE INVENTION

The object of the invention was to disclose a pressure fitting that evenat a slight pressure and in a pressureless state is movable and can beheld in an opened position.

This object is attained in accordance with a first embodiment of theinvention in that the sliding element has a piston, which can be guidedin a guide and has means for an engagement of the locking element; thatthe locking element has at least one locking pawl that is pivotableabout a pivot point; and that a damping chamber is disposed between thepiston and the guide.

Once the flow channel has opened, the grip and hold of the valve coneassure that the flow channel remains open upon a pressure relief, evenif the pressure relief results in a pressureless state. The valve coneis securely held in a position that opens the flow channel by thelocking device. This is especially advantageous for a nuclear reactorpressure vessel, since a pressure relief down to a very low pressure canthus be carried out, making it simpler to perform safety measures,especially cooling of the reactor core, to protect the reactor core.Even in an extremely unlikely case of a meltdown of the nuclear reactor,an effective pressure reduction to a low pressure is assured, so thatall the safety measures can be employed without problems andeffectively. Moreover, a pressure relief operation can be initiated andcarried out with the pressure fitting in a controlled manner, optionallyby hand.

The damping chamber prevents a hard impact of the valve cone and thepiston when the valve cone takes its seat as the pressure fittingcloses. The damping chamber also slows down the motion of the pistonupon opening of the pressure fitting, especially because the pressureincrease occurs only gradually. This also lessens mechanical strain onthe pressure fitting caused by any collision of components.

In a second embodiment of the invention, the object is attained in thata control device is provided, with which the locking device is movableunder control into the locking position and out of that position into afree position, in which the valve cone is again freely displaceable.

Regardless of a pressure prevailing in the interior of the pressurefitting, it can thus be attained that a pressure relief can be carriedout by means of the pressure fitting at any time. Moreover, closure ofthe fitting is assured regardless of an internal pressure in thepressure fitting. It is also assured that the fitting can be kept openfor a pressure relief down to a pressureless state.

The piston that can be guided in a guide can also be disposed outsidethe flow channel; this prevents structural influence of the flow channeland assures an effective mode of operation of the pressure fitting. Thelocking pawl can be mounted in a simple way in the pressure fitting andcan move simply into the locking position and out of it by rotationabout a pivot point, for instance. It may be embodied as a simplemechanical element, which is reliably functional even at a high pressureand a high temperature. Depending on the operating conditions required,it may be manufactured from a suitable material, such as a stainlesssteel. For motion into the locking position, a compression spring may beprovided. For reliable locking of the valve cone, it may also beadvantageous to provide a plurality of locking pawls.

The piston preferably has a groove for engagement by the locking pawl.Mechanical engagement of the groove by the locking pawl can be carriedout reliably even at a high pressure and temperature, so that opening ofthe pressure fitting, and keeping it open, is assured in every case.

The control valve can preferably be operated via a motor. The motor maybe switched under remote control to bring about a controlled pressurerelief.

Preferably, the motor drives a tappet, which particularly via aswitching sheath moves the locking device into and out of the lockingposition. When the tappet is released under the control of the motor,the locking device is moved into the locking position. A contrary motionof the tappet moves the locking device back out of the locking position.

Advantageously, the control valve may be driven via two mutuallyindependent motors. This makes it possible for each of the motors to bechecked and possible replaced independently of the other one duringoperation. This is because only a single motor needs to be functionalfor opening, holding open, or closing the pressure fitting. This notonly increases the safety of a plant with components that are underpressure, but a redundant embodiment also makes maintenance easier andlessens the likelihood of malfunction of the pressure fitting.

Preferably, an opening and a closing of the flow channel can be carriedout, particularly for a pressure relief, via the control device. Such anopening and closing of the flow channel, or in other words acorresponding motion of the valve cone, is preferably controlled via atleast one relief channel, which communicates with the pressure fittingand is closable by a control valve cone of the control valve. An openingof the relief channel causes a motion of the valve cone that opens theflow channel, and closing of the relief channel causes a closing motion.

Preferably, the pressure fitting operates by the relief principle inwhich with the flow channel closed, a pressure can be brought to bearupstream of the valve cone, which pressure is reducible in the interiorof the pressure fitting, by which means a motion of the valve cone thatopens the flow channel is generated. In the closing position of thepressure fitting, for instance, the pressure of a system that is underpressure and is to be relieved may prevail in the interior of thepressure fitting. In particular, a force is exerted on the valve conevia the piston so that the valve cone closes the flow channel. The forcemay be generated both via the pressure and via an additional springforce. If the pressure in the interior of the pressure fittingdecreases, the force on the valve cone or piston decreases, and alifting force caused by the pressure and engaging the underside of thepiston increases, which leads to a motion of the valve cone or pistonthat opens the flow channel. Upon a closure of the relief channel, a newpressure buildup takes place in the interior of the pressure fitting, sothat the force acting on the valve cone or the piston increases,resulting in a re-closure of the flow channel.

Preferably, the pressure fitting has a control valve which is internallydrivable with system-internal medium, which if a critical pressure inthe interior of the pressure fitting is exceeded causes an opening ofthe flow channel, and if the pressure is below the critical pressureeffects a reclosure of the flow channel. The control valve may be aspring valve, which communicates with the interior of the pressurefitting via a further relief channel. If the pressure in the interior ofthe pressure fitting rises, then the pressure applied to the internallydrivable control valve rises as well. If that pressure reaches acritical value, which leads to a force that exceeds the spring force,the result is opening of the control valve. Hence a pressure relief ofthe interior of the pressure fitting takes place, as a result of whichan opening of the flow channel is attained and thus a pressure relief ofthe pressurized system is accomplished. If the pressure in the interiorof the pressure fitting drops, the force exerted on the control valvedrops, so that beyond a certain pressure value the control valve closesagain the result of which is again a re-closure of the pressure fitting.Any further pressure relief of the pressurized system is thus prevented.The pressure fitting is accordingly suitable as a safety valve for apressurized system, as well, especially for a pressure vessel of apressurized water reactor.

Preferably, the valve cone in the downstream direction is approximatelyonion-shaped. Together with a suitable shape of the flow channel, thisallows an especially favorable flow guidance and hence an especiallycompact embodiment of the pressure fitting. The valve cone is preferablyembodied elastically in the manner of onion peels, which assuresparticularly good seat tightness, even when the geometry of the valveseat, on which the valve cone in the flow cone rests, is thermally ormechanically deformed. The especially compact embodiment thus attainedis thus not only economical but also has a positive effect on thedynamics on the pressure fitting, especially because the masses to bemoved are smaller and hence lesser forces arise. Moreover, lesser impactstrains, particular on the valve cone, occur, and simpler holding of thepressure fitting is possible.

Preferably, a throttle channel is provided in the guide, connecting thedamping chamber with the flow channel. This assures that a medium, suchas water steam, located in the flow channel will flow in throttledfashion into the damping chamber. Only a gradual pressure rise thusoccurs in the damping chamber upon opening of the pressure fitting. Thisslows down a motion of the piston, especially an upward motion along themain axis of the pressure fitting. The danger of excessive mechanicalstrain on the pressure fitting is thus further reduced.

For closure of the pressure fitting from a position that opens the flowchannel, and especially for re-closure, the pressure fitting preferablyhas a control chamber, which is located on the side of the pistonopposite the valve cone and communicates with the flow channel via athrottle channel. The control chamber can be pressure-relieved via arelief channel, which can be opened and closed via an externally drivencontrol valve for instance. Once the relief channel is closed, thecontrol chamber is filled with medium from the flow channel via thethrottle channel. This causes a pressure buildup inside the controlchamber, and the piston is moved hydraulically along the main axis,particularly downward, if the locking element is held outside itslocking position, for instance via the externally drivable controlvalve. The valve cone is thus brought to a position that closes the flowchannel. If the flow channel is already closed, then the medium locatedin the controlled chamber keeps it closed.

A use of the pressure fitting in the form of a pressure relief valve fora pressure vessel of a nuclear reactor, in particular in a pressurizedwater reactor is especially advantageous, since with the pressurefitting, a pressure relief of the pressure vessel down to a lowpressure, in particular approximately 2 to 4 bar or less, is assured.

The pressure fitting is also suitable for use as a combined pressurerelief and safety valve for a pressure vessel of a nuclear reactor, inparticular in a pressurized water reactor. The term "pressure relief" isunderstood here to mean the lowering of the pressure in the interior ofa pressure vessel to below the normal operating pressure, and "safety"is understood to mean securing a pressure vessel, which is underinternal pressure, against overpressure. A combined pressure relief andsafety valve thus has both properties, namely of opening if a criticalpressure is exceeded and thus reducing any further dangerous pressurerise, and upon a corresponding demand, by opening, of reducing theinternal pressure inside the vessel to a desired amount, and inparticular to the ambient pressure. Via the externally drivable controlvalve, a pressure relief is possible regardless of the pressureprevailing in the interior of the pressure fitting. Because of thelocking element, a pressure relief down to a very low pressure, even toa pressureless state, is made possible. This is advantageous forinstance when the pressure vessel is inspected. Via the externallydrivable control valve, a re-closure of the pressure fitting can also becarried out. To that end, if necessary, the locking element is movedback out of its locking position. By means of the internally drivablecontrol valve, in particular a spring valve, the pressure fittingmoreover performs the function of a safety valve. The pressure fittingthus represents an especially compact combined pressure relief andsafety valve, which can be kept open down to a pressureless state.

Preferably, the pressure fitting is used as a combined pressure reliefand safety valve for a pressurizer of a pressurized water reactor.

The pressure fitting will be described in further detail in conjunctionwith the drawing. The various drawing figures, each in a longitudinalsection, show the following:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, a pressure fitting, which is connected to a pressure vessel;

FIG. 2, an enlarged view of the pressure fitting in the closed state;

FIG. 3, an enlarged view of the pressure fitting in the open state;

FIG. 4, an enlarged view of the pressure fitting in the open state.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Each of the drawing figures shows only those components of the pressurefitting essential for purposes of explanation.

In FIG. 1, a pressure fitting 1 is shown, which is shown on a domeattachment 49 of a pressurizer 47, for instance of a pressurized waterreactor. Adjoining it under the pressure fitting 1 is a pressure reliefpipe 48 along a main axis 41 of the pressure fitting 1. An externallydrivable control valve 16 with two motors 46 is disposed on a lid 2 ofthe pressure fitting 1. A further pressure relief pipe 48 extends awayfrom the control valve 16. On a side of the pressure fitting 1 oppositethe dome attachment 49, an internally drivable control valve 19, whichis a spring valve and likewise has a pressure relief pipe 48, isprovided.

FIG. 2 shows a section through the pressure fitting in the closed state.The pressure fitting 1 has a flow channel 42, which extends along anaxis 45 into the pressure fitting 1 and leads out of the pressurefitting along the main axis 41. A valve cone 5 of the pressure fitting 1is disposed on one end 44 of a sliding element 43, which is a pistonrod. The sliding element 43 and hence the valve cone 5 as well aredisplaceable along the main axis 41. The valve cone 5 is seated on avalve seat 6 and closes off the flow channel 42 in the direction of themain axis 41. The main axis 41 and the axis 45 of the incoming flowchannel 42 are perpendicular to one another. In the portion of the flowchannel 42 that extends parallel to the axis 45, or in other words thehorizontal portion, a pressurized medium, such as saturated steam, ispresent. In the region of the flow channel 42 that extends parallel tothe main axis 41, or in other words vertically, a lesser pressureprevails. The valve cone 5, with its upper side 5a, faces toward a guide7 of the sliding element 43.

Above the flow channel 42, the sliding element 43 has a piston 4, whichis guided in a guide cylinder 3 and is sealed off from it by pistonrings, not identified by reference numerals.

The piston 4 thus divides the interior of the pressure fitting 1 into acylindrical damping chamber 26, located below the piston 4 and orientedtoward the valve cone 5, and a control chamber 22 that is located abovethe piston 4. A locking device 12, which is a locking pawl, isrotationally movably disposed about a pivot point inside the controlchamber 22. The piston 4 is pressed downward, that is, in the directionof the valve cone 5, by a valve spring 8. The locking pawl is kept outof the locking position via a switching sheath 10. A spring 11 of theswitching sheath 10 is held in a spring carrier 9 and exerts anupward-directed force upon the switching sheath 10. Counter to thisforce, the switching sheath 10 is kept in its position via tappets 14,15. The tappets 14, 15 are in turn held via respective control valvecones 20, 21 of the motor-driven control valve 16. From the controlchamber 22, a pressure relief channel 23, 23a leads away in onedirection to the motor-driven control valve 16 and in the other to theinternally drivable control valve 19, that is, the spring valve. Otherpressure relief channels 24, 25 connect the control chamber 22 to themotor-driven control valve 16.

On its side toward the locking pawl 12, the piston 4 has an encompassingcollar, embodied as a groove 4a, which can be engaged by the lockingpawl 12 in its locking position. The pressure fitting 1 is in its normaloperating state; that is, both the spring valve 19 and the two controlvalve cones 20, 21 of the motor-drivable control valve 16 are closed.The valve cone 5 is also located on its valve seat 6 and closes off theflow channel 42. The control valve cones 20, 21, via the tappets 14, 15,press the switching sheath 10 downward counter to the force of thespring 11. As a result, the lower end 10a of the switching sheath 10 ispressed against a cam 12a of the locking pawl 12, which in turn is movedout of the region of the groove 4a of the piston 4. In this position ofthe locking pawl 12, the pressure fitting 1 can be used as a safetyvalve unrestrictedly for pressure relief and pressure securing.

In FIG. 3, the pressure fitting 1 is shown in the open state in itsfunction as a pressure securing or safety valve. At a critical systempressure, such as 176 bar, the spring valve 19 has relieved the pressureof the control chamber 22 of the pressure fitting 1 via the reliefchannel 23, 23a to such an extent that the valve cone 5 has reached aposition that opens the flow channel 42. The valve cone 5 rests with itstop side 5a against the guide 7. In accordance with the motion of thevalve cone 5, the piston 4 has been moved upward as well. It is locatedin the immediate vicinity of the locking pawl 12, but without engagementby the pawl of the intended groove 4a of the piston 4. The switchingsheath 10 with its spring 11 is held by the tappet 14 in such a way thatit keeps the locking pawl 12 out of the locking position. The dampingchamber 26 between the piston 4 and the guide 7 is at its maximum sizein this position. It is sealed off from the control chamber 22 by thepiston 4, and it communicates with the flow channel 42 via a throttlegap 28, which extends parallel to the main axis 41 between the slidingelement 43 and the guide 7. Both in an opening motion of the piston 4and a closing motion of the piston 4, the damping chamber 26 causes acorresponding slowing of the motion of the piston 4. Upon a pressurereduction in the control chamber 22, the piston 4 is moved upwardcounter to the closing force, which is generated in particular via thevalve spring 8 and the pressure in the control chamber 22, in responseto an imposition of pressure on the damping chamber 26. To avoid anoverly rapid opening motion and hence a strong dynamic load, throttlingof the pressure relief of the control chamber 22 is provided. This isattained in part by the damping chamber 26, which increases in sizesteadily upon an upward motion of the piston, but in which a pressurebuildup occurs only in delayed fashion because of the throttle gap 28.Throttling of the pressure relief of the control chamber 22 is moreoverattained by the fact that the sliding element 43 has a control edge 30above the piston 4. This control edge 30, beyond a predetermined strokeof the piston 4, closes a pressure relief opening 29 in an upper guide7b surrounding the sliding element 43, and as a result the pressurerelief is lessened. If the piston 4 has assumed a top terminal position,that is, if the valve cone 5 rests with its top side 5a on the guide 7,then a pressure equalization takes place between the damping chamber 26and the flow channel 42.

In a closing operation of the pressure fitting 1, an additionalhydraulic force acts on the annular top side 5a of the valve cone 5. Anacceleration of the valve cone 5 by this hydraulic force can likewise beprevented by the damping chamber 26, since the medium located in thedamping chamber 26 must flow through the likewise annular throttle gap28 into the flow channel 42. Upon a rapid downward motion of the piston4, the medium is thus unable to flow unhindered out of the dampingchamber 26, so that a compression of the medium and hence an increase inpressure takes place in the damping chamber 26. This pressure increasecounteracts the downward motion of the piston 4 and slows down theclosing operation of the pressure fitting 1. This largely prevents ahard impact of the valve cone 5 as it meets its seat 6.

The closing operation of the pressure fitting 1 begins as soon as thespring valve 19 closes again in response to a dropping pressure, forinstance to below about 150 bar. This terminates any further relief ofthe pressure fitting 1, and particularly of the control chamber 22. Viathe throttle channel 4b, medium flows out of the flow channel 42 intothe control chamber 22 to replenish it and causes a correspondingpressure imposition on the piston 4. This produces a re-closure of thepressure fitting 1, without hindrance from the locking device 12. Thepressure fitting 1 thus acts as a pressure securing or safety valve.

In FIG. 4, the pressure fitting 1 is shown in the open state in itsfunction as a pressure relief valve. To illustrate the engagement of thelocking pawl 12 with the groove 4a of the piston 4, only a portion ofthe pressure fitting between the main axis 41 and the spring valve 19 isshown. Via the respective motors 46 (see FIG. 1), the control valvecones 20, 21 have been moved out of their respective seat 27 so that viathe relief channels 23, 24 and 25, a pressure relief of the controlchamber 22 has taken place. Both the piston 4 and the valve cone 5 havethus been moved hydraulically upward, that is, away from the valve seat6. The switching sheath 10 and the tappets 14, 15 have also been movedupward by the spring 11, so that the locking pawl 12 has been rotatedinto the locking position by a compression spring 13. The pawl engagesthe groove 4a, which is the encompassing collar of the piston 4, andretains the piston, should it drop, in a position that opens the flowchannel.

With the motor-drivable control valve 16 open and with adequately highpressure in the damping chamber 26, the valve cone 5 and the piston 4remain in the position shown, which opens the flow channel. If thepressure drops below a minimum value, then the piston 4 begins to movedownward both by the weight of the sliding element 43 and by the forceexerted by the valve spring 8. This motion, and the correspondingforces, are absorbed by the locking pawl 12, and both the piston 4 andthe valve cone 5 are held securely in the position that opens the flowchannel. To that end, it is also possible for a plurality of lockingpawls to be provided.

Functional testing of the pressure fitting 1 during operation of thepressurized system, especially a nuclear reactor pressure vessel, can becarried out in a simple way, for instance at a reduced pressure of about40 bar, when the system is put online or taken offline. Opening of themotor-drivable control valve 16 leads to a hydraulic opening of thepressure fitting 1 and a motion of the locking pawl or pawls 12 into thelocking position. A position indication of the piston 4 is provided viaa position indicator rod 18, which is guided in a thimble tube 17 and isconnected with the sliding element 43 along the main axis 41. If in anensuing re-closing operation one of the two control valve cones 20, 21fails to reach its valve seat 21 or remains entirely open, closure ofthe pressure fitting 1 takes place nevertheless. This is because asingle closing control valve cone 20, 21 moves the locking pawl 12 outof the groove 4a and causes a closure of the relief channels 23, 24, 25,resulting in a hydraulic closure of the pressure fitting 1. Replacementof a defective motor 46 of the motor-drivable control valve 16 is thuspossible even during operation of the system, which in particular is anuclear reactor, without having to shut down the system. By moving thecontrol valve cones 20, 21 in alternation out of their respective valveseat 27, the functional capability of the motor-drivable control valve16 can be checked even during full-load operation of the system, withoutcausing the flow channel 42 to open. The locking pawls 12 remains insuch a test always out of the locking position, so that the pressurefitting 1 remains constantly ready for use as a pressure securing valveduring the test.

The invention is distinguished by the fact that with a single, extremelycompact pressure fitting, a combined pressure relief and safety valve isprovided, which assures a pressure relief down to a pressureless state.A closing motion of the valve cone is reliably prevented via a lockingdevice, especially a locking pawl; the locking device engages a slidingelement and thus holds the valve cone firmly in a position that opensthe flow channel. By means of an externally drivable control valve withat least two mutually independent motors, checking of this control valvecan be done even during operation of a pressurized system. The pressurefitting is especially suitable as a combined pressure relief and safetyvalve for a pressurizer of a pressurized water reactor.

We claim:
 1. A pressure fitting with a flow channel, comprising:asliding element disposed displaceably along a main axis of the pressurefitting, said sliding element having an end; a valve cone disposed onsaid end of said sliding element, said valve cone being selectivelymovable between an open valve position in which a flow channel of thepressure fitting is open, and a closed valve position in which the flowchannel is closed; a locking element engaging said sliding element in anopen valve position of said valve cone, for maintaining the open valveposition; a guide slidably guiding said sliding element, said slidingelement including a piston adapted to be engaged by said lockingelement; and said locking element having at least one locking pawl beingpivotable about a pivot point; and said piston and said guide enclosinga damping chamber therebetween.
 2. The pressure fitting according toclaim 1, which operates by relief principle, wherein, in the closedvalve position, a pressure can be brought to bear upstream of said valvecone, which pressure is reducible in an interior of said pressurefitting, whereby said valve cone is moved towards the open valveposition.
 3. The pressure fitting according to claim 2, which furthercomprises an internally drivable control valve, said control valveopening said flow channel when a critical pressure in the interior ofthe pressure fitting is exceeded, and said control valve closing saidflow channel when the pressure falls below the critical pressure.
 4. Thepressure fitting according to claim 1, wherein said valve cone isapproximately onion-shaped in a direction opposite a flow through theflow channel.
 5. The pressure fitting according to claim 1, wherein saidpiston has a groove formed therein for engagement by said at least onelocking pawl.
 6. The pressure fitting according to claim 1, wherein saidpiston and a lower region of said guide define a damping chambertherebetween.
 7. The pressure fitting according to claim 6, wherein saidguide has a throttle conduit formed therein, said throttle conduitconnecting said damping chamber with said flow channel.
 8. The pressurefitting according to claim 1, wherein a control chamber formed on a sideof said piston opposite said valve cone, said control chambercommunicating with said flow channel via a throttle conduit.
 9. Thepressure fitting according to claim 1, wherein the pressure fitting is apressure relief valve for a pressure vessel of a nuclear reactor. 10.The pressure fitting according to claim 9, wherein, the nuclear reactoris a pressurized water reactor.
 11. The pressure fitting according toclaim 1, wherein the pressure fitting is a combined pressure reliefvalve and safety valve for a pressure vessel of a nuclear reactor. 12.The pressure fitting according to claim 1, wherein the pressure fittingis a combined pressure relief valve and safety valve for a pressurizerof a pressurized water reactor.
 13. A pressure fitting with a flowchannel, comprising:a sliding element disposed displaceably along a mainaxis of the pressure fitting, said sliding element having an end; avalve cone disposed on said end of said sliding element, said valve conebeing selectively movable between an open valve position in which a flowchannel of the pressure fitting open, and a closed valve position inwhich the flow channel is closed; a locking element engaging saidsliding element in an open valve position of said valve cone, formaintaining the open valve position; and a control device operativelyconnected with said locking element, said control device moving saidlocking element between a catch position in which said locking elementmaintains the open valve position and a free position, in which thevalve cone is freely displaceable.
 14. The pressure fitting according toclaim 13, wherein said control device is a control valve, and includingat least one motor driving said control valve.
 15. The pressure fittingaccording to claim 14, which further comprises a tappet connected tosaid locking element, said at least one motor driving said tappet formoving said locking element into and out of the catch position.
 16. Thepressure fitting according to claim 15, which further comprises aswitching sheath disposed between said locking element and said tappet.17. The pressure fitting according to claim 14, wherein said at leastone motor is one of two mutually independent motors, and said controlvalve is drivable via said two independent motors.
 18. The pressurefitting according to claim 13, wherein said control device is a controlvalve drivable with external medium.
 19. The pressure fitting accordingto claim 13, wherein said control device causes an opening and aclosing, respectively, of the flow channel for a pressure reliefoperation.
 20. The pressure fitting according to claim 13, whichoperates by relief principle, wherein, in the closed valve position, apressure can be brought to bear upstream of said valve cone, whichpressure is reducible in an interior of said pressure fitting, wherebysaid valve cone is moved towards the open valve position.
 21. Thepressure fitting according to claim 20, which further comprises aninternally drivable control valve, said control valve opening said flowchannel when a critical pressure in the interior of the pressure fittingis exceeded, and said control valve closing said flow channel when thepressure falls below the critical pressure.
 22. The pressure fittingaccording to claim 13, wherein said valve cone is approximatelyonion-shaped in a direction opposite a flow through the flow channel.23. The pressure fitting according to claim 13, wherein the pressurefitting is a pressure relief valve for a pressure vessel of a nuclearreactor.
 24. The pressure fitting according to claim 23, wherein, thenuclear reactor is a pressurized water reactor.
 25. The pressure fittingaccording to claim 13, wherein the pressure fitting is a combinedpressure relief valve and safety valve for a pressure vessel of anuclear reactor.
 26. The pressure fitting according to claim 13, whereinthe pressure fitting is a combined pressure relief valve and safetyvalve for a pressurizer of a pressurized water reactor.